JP2009068079A - Steel tube with excellent steam oxidation resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel tube having excellent steam oxidation resistance and suitable as a material used for boilers in power generation facilities, tubing and various heat exchangers, etc. <P>SOLUTION: The steel tube with excellent steam oxidation resistance contains 8 to 28 mass% Cr and has a worked layer in the inner surface thereof and satisfies inequality α≥0.5 (wherein α is represented by equation α=äHv<SB>20</SB>-Hv<SB>t/2</SB>}/Hv<SB>t/2</SB>). In the equation, Hv<SB>20</SB>and Hv<SB>t/2</SB>denote a Vickers hardness in a position at a depth of 20 μm from the inner surface of the steel tube and a Vickers hardness in a position at a depth of t/2 (where (t) is wall thickness of the steel tube) from the inner surface of the steel tube, respectively. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a steel pipe having steam oxidation resistance, and more particularly to a steel pipe having excellent steam oxidation resistance suitable for materials used for boilers, piping, various heat exchangers and the like of power generation equipment.

  In recent years, interest in global environmental problems such as global warming is increasing in various technical fields. For example, in a high power generation plant, it is an urgent need to suppress the total discharge amount of carbon dioxide gas, and a newly constructed plant is strongly required to be a facility capable of generating power with high efficiency. For example, in a thermal power generation boiler, high-temperature and high-pressure steam is adopted as an effective measure for high-efficiency power generation. This increase in the temperature and pressure of steam causes an increase in the wall temperature of the superheater pipe and reheater pipe of the boiler, and the boiler steel pipe used is required to have high-temperature strength and resistance to high-temperature oxidation by steam. As a method for preventing steam oxidation of a steel pipe, various proposals have been made so far as shown below.

(A) Technology for performing solution heat treatment after forming a processed layer In Patent Document 1, after austenitic stainless steel is subjected to solution treatment, cold processing such as shot processing, grinder processing and polishing processing is added to the tube surface. Then, an invention relating to a method for producing a surface fine-grained stainless steel pipe that is subjected to a predetermined re-solution treatment is disclosed. Patent Document 2 discloses that austenitic stainless steel pipe is subjected to cold working with a working rate of 20% or more, and then 2 An invention relating to a heat treatment method for performing a solution heat treatment at a temperature rising rate of 9 ° C./sec or less is disclosed.

  In Patent Document 3, the crystal grain size No. An invention relating to a manufacturing method of an austenitic iron alloy tube is disclosed in which a fine-grained layer having a thickness of 30 μm or more is formed from No. 7 and subjected to a recrystallization treatment by subjecting it to a cold working of 20% or more. Patent Document 4 discloses an austenitic stainless steel pipe that is cold-worked so that the hardness at a position of 20 μm from the inner surface is Hv320 or more and that is subjected to a solution treatment in a state where the chemically affected surface is reduced as much as possible. An invention relating to a method for improving the steam oxidation resistance of a rubber is disclosed.

(B) Technology for increasing the content of C and N to make a fine grain structure even after solution heat treatment In Patent Document 5, the crystal grain size number is No. on the inner surface of the steel pipe. An invention relating to an austenitic stainless steel pipe having a fine grain structure of 7 or more and having C + N of the fine grain layer part of 0.15% or more is disclosed.

(C) Technology for forming a cold-worked layer by peening process Patent Document 6 discloses an invention relating to a method for preventing high-temperature steam oxidation of an austenitic stainless steel pipe that is subjected to peening by particle spraying on the inner surface after the final heat treatment. Patent Document 7 discloses an invention relating to a method for preventing high temperature steam oxidation of an austenitic stainless steel pipe, characterized in that a processed layer of 10 μm or more is formed by selecting a particle spraying condition for peening. . Further, Patent Document 8 discloses that a tubular body taken out from an existing boiler is subjected to chemical cleaning for the purpose of descaling the inner surface after heat treatment, and then a shot intended to form a cold-worked layer on the inner surface of the tubular body. An invention relating to a method of treating a stainless steel tube of an existing boiler that performs blasting is disclosed.

(D) Technology for improving scale adhesion Patent Document 9 describes excellent steam oxidation resistance having a particle sprayed peening layer on the inner surface of a solution-treated steel pipe made of austenitic stainless steel containing a rare earth element. An invention related to a steel pipe for a boiler is disclosed. Patent Document 10 contains 9 to 28% by mass of Cr, the maximum height of the inner surface after cold working is 15 μm or more, and the Vickers hardness of the inner surface layer of the tube and the center of the wall thickness. An invention relating to a steel pipe having a difference of 100 or more is disclosed.

(E) Technology for imparting cold work with a high degree of work In Patent Document 11, ultrasonic shock treatment is applied to the inner surface of a ferritic heat-resistant steel pipe or austenitic heat-resistant steel pipe containing 5 to 30% Cr by mass%. The invention regarding the manufacturing method of the steel pipe for boilers which was excellent in the steam oxidation resistance to give is indicated.

(F) Technology for improving steam oxidation resistance of ferritic heat resistant steel Patent Document 12 discloses that a steel containing Ti and Y is quenched and tempered, and has a diameter of 1 μm or less at the interface with the oxide film or in the vicinity thereof. An invention relating to a high Cr ferritic heat resistant steel in which an oxide is formed is disclosed, and Patent Document 13 discloses that at least a region having a surface depth of 10 μm has a processed structure composed of elongated ferrite grains or a ferrite based grain size. Discloses an invention relating to a high Cr ferritic heat resistant steel having a microstructure of 3 μm or less and having a protective film on the surface.

  Patent Document 14 discloses a ferrite system in which a steel having a Cr content of 9.5 to 15% is normalized and tempered to uniformize crystal grains and structures and then sprayed onto the surface to form a shot processed layer. An invention relating to a heat-resistant steel processing method is disclosed, and Patent Document 15 discloses a shot peening treatment using a powder that forms a protective film with excellent oxidation resistance on the surface of a high Cr ferritic heat-resistant steel as a shot material. An invention relating to a surface treatment method in which a pre-oxidation treatment is performed to form an oxide protective film on the steel surface is disclosed. Patent Document 16 discloses an improvement method characterized by supporting Cr powder particles on the surface of a ferritic heat-resistant steel containing Cr to produce a Cr oxide layer having a high Cr concentration at a high temperature. Is disclosed.

JP-A-53-114722 JP 54-138814 A JP-A-55-58329 JP 58-39733 A JP 58-133352 A JP-A-49-135822 JP 52-8930 A JP-A-63-54598 JP-A-6-322489 JP 2006-307313 A JP 2004-132437 A JP-A-11-92880 Japanese Patent Laid-Open No. 2004-156075 JP 2002-285236 A JP 2005-298878 A JP 2007-39745 A

  The technique shown in the above (A) is a solution heat treatment at a high temperature in order to improve the decrease in creep rupture strength and stress corrosion cracking. However, the solution heat treatment removes strain introduced into the crystal grains by processing together with the above improvement effect, and recrystallization occurs. Depending on the chemical composition of the steel pipe, the crystal grains grown by the solution treatment become too large, and it becomes difficult to stably maintain the fine grain layer on the steel surface. As a result, the steam oxidation resistance improved by cold working may be reduced.

  Although the technique shown in (B) can slightly improve the resistance of the pipe to steam oxidation, the surface layer in the pipe is extremely sensitized during use of the boiler. There is a risk of occurrence.

  As described above, the techniques shown in (A) and (B) both include many problems in practical use.

  The technology shown in (C) is positioned as one of the effective technologies for preventing steam oxidation in current commercial boilers, that is, boilers having a steam temperature of 566 ° C. (1050 ° F.). It is applied to a part of steel pipes for boilers. However, for example, at a steam temperature of a high-efficiency boiler employed in a new plant of 621 ° C. (1150 ° F.), the wall temperature of the superheater pipe and the reheater pipe is 50 to 100 ° C. higher than that of the current boiler. Become. Boilers with operating conditions with higher steam temperatures such as 650 ° C. and 700 ° C. are also being studied. In such a high temperature range, it is difficult to maintain the effect of inhibiting steam oxidation by the technique shown in (C) for a long time.

  According to the technique shown in (D), the protective property of the scale can be extended and maintained, but it cannot be said that it is sufficient in consideration of the usage time of the steel pipe. In addition, the technique shown in (E) can provide cold work with a high degree of work, which is effective for initial scale formation, but has a problem in maintaining for a long time. leave. There is also a problem of introduction and operation cost of the ultrasonic impact treatment apparatus.

  The technique shown in (F) can form a protective film with high adhesion on a ferritic heat resistant steel in which a Cr oxide film is difficult to be formed. However, it has not reached an essential solution considering long-term use.

  The present invention has been made to solve the above-described problems of the prior art, and an object of the present invention is to provide a steel pipe having extremely excellent steam oxidation resistance.

  As a result of intensive studies to investigate the essential problems in steel pipes that maintain steam oxidation resistance, the present inventors have obtained the following knowledge.

(A) When the surface of a material that has not been processed after the solution heat treatment, that is, the material that has been subjected to the solution heat treatment, is brought into contact with high-temperature steam, a thick steam oxide scale called a two-layer scale is formed on the surface. On the other hand, when a material processed by solution heat treatment is brought into contact with high-temperature water vapor, a Cr oxide scale having a slow growth rate such as Cr ₂ O ₃ is extremely thin and uniformly formed on the steel surface.

  (B) Since this Cr oxide scale having a high protective property can exist stably for a long time at a material temperature of 600 ° C. or lower, the generation of a two-layer scale is hardly observed on the material surface. However, in a high temperature range where the material temperature exceeds 600 ° C., even if the Cr oxide scale is uniformly generated in the initial stage of steam oxidation, abnormal oxidation occurs from a weak spot partially during long-time use. In particular, a thick scale covers the entire material surface.

  (C) Although the occurrence of defects such as peeling and cracking of Cr oxide scale can be reduced to some extent by a conventionally known method, it is difficult to completely suppress this.

  (D) Even if a defect occurs in the Cr oxide scale, abnormal oxidation does not occur if so-called “repair” occurs in which a protective oxide scale is re-formed at that location. In other words, if a repair function is given to the processed layer formed on the steel surface, the steam oxidation resistance can be maintained for a long time.

  The inventors of the present invention further advanced research focusing on the structural change caused by heating of the processed layer on the steel surface, and obtained the following new knowledge.

  (E) When the degree of processing applied to the surface of the steel pipe is small, so-called recovery in which strain disappears during use at a high temperature occurs, and the crystal grain size does not change. At this time, the processed layer directly under the Cr oxide scale changes to a texture state close to the surface of the material as it is as a solution treatment that is not processed. Such a processed layer is insufficiently repaired when peeling or the like occurs in the oxide scale.

  (F) When the degree of processing applied to the steel pipe surface is increased, nucleation and growth, that is, recrystallization occurs during use at a high temperature. Since this recrystallized structure has a very fine grain size, the grain boundary diffusion of Cr becomes easy. As a result, the Cr necessary for forming the Cr oxide scale can be sufficiently supplied from the inside of the material, so that the repair is possible.

  (G) When the degree of processing applied to the steel pipe surface is high, a fine recrystallized structure is formed on the outermost surface of the material. The reason why such a fine grain structure is formed in the processing stage is not clear, but the driving force for recrystallization is increased by adding very large processing, and recrystallization that usually occurs at high temperatures occurs even at low temperatures. It is considered a thing. Since this structure consists of very fine crystal grains, the grain boundary diffusion of Cr is facilitated, and the uniformity of Cr oxide scale formation in the initial stage of heating is further enhanced. As a result, there is an effect that it is possible to reduce weak spots that become defects such as peeling.

  As described above, in order to maintain the steam oxidation resistance of the steel pipe at a high temperature for a long time, a protective oxide scale is uniformly formed in the initial stage to suppress the scale peeling and the scale peeling occurs temporarily. It is necessary to be able to repair even if it is.

  The present invention has been made on the basis of such findings, and the gist thereof is the steel pipe having excellent steam oxidation resistance (i) to (v) below.

(i) A steel pipe containing 8 to 28% by mass of Cr and having a processed layer on its inner surface and excellent in steam oxidation resistance satisfying the following formula (1).
α ≧ 0.5 (1)
However, (alpha) in (1) Formula is shown by a following formula.
α = {Hv ₂₀ −Hv _{t / 2} } / Hv _{t / 2}
Here, Hv ₂₀ is Vickers hardness at a position where the depth from the inner surface of the steel pipe is 20 μm, and Hv _{t / 2} is a position where the depth from the inner surface of the steel pipe is t / 2 (t: the thickness of the steel pipe). Each means Vickers hardness.

  (ii) The steel pipe excellent in steam oxidation resistance as described in (i) above, wherein the processed layer has a crystal structure having a crystal grain size of 1 μm or less at a depth of 1 μm from the surface.

  (iii) The steel pipe having excellent steam oxidation resistance according to the above (i) or (ii), wherein the steel pipe is composed of a ferritic heat-resistant steel pipe containing 8 to 28% by mass of Cr.

  (iv) The steel pipe is composed of austenitic stainless steel containing 15% to 28% Cr and 6% to 30% by mass%, and has excellent steam oxidation resistance as described in (i) or (ii) above. Steel pipe.

  (v) The steel pipe excellent in steam oxidation resistance according to any one of (i) to (iv) above, wherein the processed layer is a processed layer formed by spraying particles.

  According to the present invention, it is possible to uniformly form a scale having excellent protective properties on the surface of the steel pipe in the initial stage of use, and it is possible to suppress the peeling of the scale and even if the peeling of the scale occurs. Since it has a repair function, abnormal oxidation is unlikely to occur even under high temperature use conditions exceeding 600 ° C.

A steel pipe having a protective function can be uniformly formed on the surface of the steel pipe at the initial stage of use, scale peeling can be suppressed, and even if scale peeling occurs, the steel pipe has a repair function. Therefore, it is necessary to form a processing layer that satisfies the following expression (1) on the inner surface of the steel pipe under controlled processing conditions.
α ≧ 0.5 (1)
However, (alpha) in (1) Formula is shown by a following formula.
α = {Hv ₂₀ −Hv _{t / 2} } / Hv _{t / 2}
Here, Hv ₂₀ is Vickers hardness at a position where the depth from the inner surface of the steel pipe is 20 μm, and Hv _{t / 2} is a position where the depth from the inner surface of the steel pipe is t / 2 (t: the thickness of the steel pipe). Each means Vickers hardness.

  “Α” defined in the above (1) is the rate of increase in hardness, and the difference between the Vickers hardness at a depth of 20 μm from the inner surface of the steel pipe and the Vickers hardness at the central part of the steel pipe thickness is the Vickers hardness at the central part of the steel pipe thickness. Divided by. It can be said that the larger the value, the larger the processing was introduced. And if (alpha) is 0.5 or more, recrystallization will arise in the processed layer in the range from a steel pipe inner surface to 20 micrometers during use at high temperature. Therefore, if α is a processed layer having a value of 0.5 or more, the peeled portion can be repaired even if peeling occurs on the protective scale during use at a high temperature.

  α is more likely to recrystallize and become finer recrystallized grains, so there is no particular upper limit. However, if the surface becomes extremely hard, problems may occur in forming, welding, etc. Is preferably 2. Moreover, the preferable minimum of (alpha) is 0.6. A more preferred lower limit is 0.7.

  As the degree of processing applied to the inner surface of the steel pipe increases, a finer crystal structure is formed. The fine crystal structure facilitates the supply of Cr to the surface at a high temperature, and particularly prevents abnormal oxidation in the initial stage of use, thereby contributing to the uniform formation of Cr oxide scale. This effect has a correlation with the crystal grain size in the surface layer portion of the processed layer. That is, when the crystal grain size at a depth of 1 μm from the surface of the processed layer is 1 μm or less, uniform formation of Cr oxide scale at the initial stage of use can be easily obtained. The crystal grain size of the fine crystals is preferably 800 nm or less, and more preferably 500 nm or less. The fine crystal structure may be formed in a range from the inner surface of the steel pipe to a depth of 1 μm, but may further be formed in a range where the depth exceeds 1 μm.

  The pipe | tube used as the object of this invention is heat-resistant steel pipes, such as an alloy steel pipe, a ferritic type, and an austenitic type. There are no particular restrictions on the specific material, but the scale generated on the inner surface of the tube must be mainly composed of Cr oxide, so the tube material is 8% to 28% Cr by mass%. It is necessary to be a steel pipe containing. The tube material is a heat-resistant steel made of ferritic alloy steel or stainless steel containing 8 to 28% by mass of Cr, or an austenitic stainless steel containing 15 to 28% by mass of Cr and 6 to 30% by mass of Ni. It is desirable that

  For example, the material of the pipe that is the subject of the present invention is STBA26 alloy steel defined by JIS standard, ferritic stainless steel such as SUS410, austenitic stainless steel such as SUS304, SUS309, SUS310, SUS316, SUS321, and SUS347. , And their equivalent steels. Examples of the chemical composition of applicable steel types are as follows. In the following description, “%” regarding the component content means “mass%”.

  (Α) Ferritic heat resistant steel containing C: 0.2% or less, Si: 2.0% or less, Mn: 0.1 to 3.0%, Cr: 8 to 28%, the balance being Fe and impurities . If necessary, this steel is Ni: 1.5% or less, Mo: 5% or less, W: 10% or less, Cu: 5% or less, N: 0.3% or less, V: 1.0% or less Nb: 1.5% or less, Ti: 0.5% or less, Ca: 0.2% or less, Mg: 0.2% or less, Al: 0.2% or less, B: 0.2% or less, rare earth Element: One or more elements selected from 0.2% or less may be contained.

  (Β) C: 0.2% or less, Si: 2.0% or less, Mn: 0.1-3.0%, Cr: 15-28%, Ni: 6-30%, the balance being Fe And austenitic stainless steel consisting of impurities. This steel has Mo: 5% or less, W: 10% or less, Cu: 5% or less, N: 0.3% or less, V: 1.0% or less, Nb: 1.5% or less as required. Ti: 0.5% or less, Ca: 0.2% or less, Mg: 0.2% or less, Al: 0.2% or less, B: 0.2% or less, rare earth element: 0.2% or less You may contain 1 or more types chosen from the inside.

  Hereafter, the effect of each component of said steel type and the reason for limitation of content are demonstrated.

C: 0.2% or less C is an element effective for securing strength and creep strength. In order to acquire the effect, it is preferable to make it contain 0.01% or more. However, if its content exceeds 0.2%, undissolved carbides remain in the solution treatment state, and may not contribute to the improvement of high temperature strength. Further, it may adversely affect mechanical properties such as toughness. Therefore, the C content is desirably 0.2% or less. In addition, from the viewpoint of hot workability and toughness deterioration, the content is preferably 0.12% or less.

Si: 2.0% or less Si is an element used as a deoxidizer, and is an element effective for improving the steam oxidation resistance. The effect becomes remarkable when it contains 0.1% or more. On the other hand, since the weldability or hot workability deteriorates when the content increases, the content is desirably 2.0% or less. A desirable content of Si is 0.8% or less.

Mn: 0.1 to 3.0%
Mn is effective as a deoxidizer in the same manner as Si. Moreover, Mn has the effect | action which suppresses deterioration of the hot workability resulting from S contained as an impurity. In order to improve the deoxidation effect and hot workability, Mn is contained in an amount of 0.1% or more. However, excessive content causes embrittlement, so the upper limit of the content is preferably 3.0%. A more desirable upper limit is 2.0%.

Cr: 8 to 28% (for ferritic heat resistant steel) or 15 to 28% (for austenitic stainless steel)
Cr is an element that contributes to high-temperature strength and is effective in improving the oxidation resistance and corrosion resistance by generating a scale mainly composed of Cr oxide on the inner surface of the steel pipe. The effect becomes remarkable when the ferritic heat-resistant steel contains 8% or more and the austenitic stainless steel contains 15% or more. However, if Cr is excessively contained, toughness and hot workability may be deteriorated. Therefore, the upper limit of the content is 28% in both cases of ferritic heat resistant steel and austenitic stainless steel. Is desirable.

Ni: 1.5% or less (for ferritic heat resistant steel) or 6 to 30% (for austenitic stainless steel)
In the case of ferritic heat resistant steel, Ni is effective in improving toughness. This effect becomes significant when the content is 0.1% or more. However, if the content exceeds 1.5%, the creep rupture strength may be reduced.

  On the other hand, in the case of austenitic stainless steel, Ni is an element necessary for stabilizing the austenitic structure and improving the creep strength. This effect becomes significant when the content is 6% or more. However, even if a large amount is added, the effect is saturated and the cost is increased, so the upper limit is desirably 30%. A preferred upper limit is 25%.

Mo: 5% or less W: 10% or less Cu: 5% or less Mo, W and Cu are preferably contained because they increase the high-temperature strength of the steel. The effect is exhibited by containing at least one of at least 0.1%. In addition, since the weldability and workability are impaired when a large amount is contained, the upper limit is 5% for Mo and Cu, and 10% for W.

N: 0.3% or less N contributes to solid solution strengthening of steel, and also has an effect of strengthening steel by precipitation strengthening action by combining with other elements. When it is desired to obtain the effect, 0.005% or more is contained. However, if it exceeds 0.3%, ductility and weldability may deteriorate.

V: 1.0% or less Nb: 1.5% or less Ti: 0.5% or less V, Nb and Ti all combine with carbon and nitrogen to form carbonitrides and contribute to precipitation strengthening Therefore, it can be added as necessary. In order to obtain this effect, it is preferable to contain 0.01% or more of one or more selected from these elements. On the other hand, if these contents are excessive, the workability of the steel may be impaired. Therefore, V is 1.0% or less, Nb is 1.5% or less, and Ti is 0.5% or less. desirable.

Ca: 0.2% or less Mg: 0.2% or less Al: 0.2% or less B: 0.2% or less Rare earth elements: 0.2% or less Ca, Mg, Al, B and rare earth elements (La, Ce , Y, Pd, Nd, etc.) all have the effect of improving strength, workability, and steam oxidation resistance, and can be added as necessary. In order to obtain these effects, it is preferable to contain 0.0001% or more of one or more selected from these elements. On the other hand, if the content of these elements exceeds 0.2%, workability or weldability may be impaired. Here, the rare earth element means 17 elements in which Y and Sc are added to 15 elements of lanthanoid.

  There is no restriction | limiting in particular about the manufacturing method of the steel pipe which concerns on this invention, A normal melting method, a casting method, and a pipe making method are employable. That is, for example, a steel having the above chemical composition is melted and cast, and then made into a raw pipe by various hot pipe making methods (extruded pipe, punched pipe, Mannesmann pipe, etc.). If necessary, softening heat treatment is applied. A hot element tube is formed into a tube having a desired shape by various cold working methods such as cold rolling and cold drawing, and then a processed layer is formed on the inner surface of the steel tube. In addition, after forming into a tube by cold working, for the purpose of homogenizing the crystal grains, the ferritic heat-resistant steel is subjected to heat treatment of normalizing or tempering after quenching, and the austenitic stainless steel is subjected to solution heat treatment. After applying, a processed layer may be formed on the inner surface of the steel pipe.

  There is no restriction | limiting in particular about the method of forming a process layer in a steel pipe inner surface. For example, various spraying methods such as known shot peening, shot blasting, shot processing, sand blasting, sand processing, air blasting, and water jet can be employed. Further, the material, shape, etc. of the particles to be sprayed are not limited. Examples of the material that can be used include steel, cast steel, stainless steel, glass, silica sand, alumina, and amorphous. As the shape, for example, a sphere, a cut wire, a grid, or the like can be used. The particles may be sprayed using compressed air, centrifugal force generated by an impeller (impeller type), high-pressure water, ultrasonic waves, or the like. Alternatively, the particles may be mixed with a liquid and sprayed with compressed air or the like (liquid honing). In addition, it is also possible to apply the processing layer by polishing, ball milling, grinder processing, honing processing, ultrasonic impact processing, or the like. In particular, when it is required to stably secure the steam oxidation resistance for a long time at a high temperature, it is preferable to form a processed layer by particle spraying that facilitates uniform processing over the entire inner surface.

  By using these methods, various conditions may be adjusted to form a processed layer that satisfies the above conditions on the inner surface of the steel pipe.

  Steel pipes having the chemical composition shown in Table 1 were prepared under various conditions, and the hardness and crystal grain size of the processed layer and the steam oxidation test were performed by the methods described below.

  Steel No. About 1, 2, and 6, after melt | dissolving in a laboratory in vacuum and manufacturing a steel pipe (outer diameter: 80 mm, wall thickness: 12 mm) by hot extrusion, the heat processing of normalization and tempering was implemented. Steel No. 3 and 5 were melted in an actual electric furnace, and a steel pipe (outer diameter: 50.8 mm, wall thickness: 8 mm) was produced by hot extrusion and cold rolling, followed by solution heat treatment. Surface treatment was performed on the inner surfaces of these steel pipes under the conditions shown in Table 2 to obtain test materials.

[Measurement of hardness of processed layer]
A 15 mm square test piece was cut out from each test material, the test piece was embedded in resin, the cross section was cut, and mirror polishing was performed. About each test piece, the Vickers hardness of the load 10g was measured in the position whose depth from the inner surface of a steel pipe is 20 micrometers, and the center position of pipe | tube thickness. Five points were measured on each specimen. The average value of the measured values was assigned to the following equation as Vickers hardness Hv ₂₀ and Hv _{t / 2} at each position, and the value of α was determined.
α = {Hv ₂₀ −Hv _{t / 2} } / Hv _{t / 2}

[Crystal grain size of processed layer]
A small test piece was cut out from each test material, and first, the surface corresponding to the cross section of the steel pipe of the test piece was observed with an optical microscope (1000 times magnification). At this time, for the test pieces (Nos. 3, 4, 10 and 11) having a crystal grain size of 1 μm or less at a depth of 1 μm from the inner surface of the steel pipe, a sample was further prepared by a field ion beam (FIB) method. The surface corresponding to the cross section of the steel pipe of each sample was observed with three fields of view with an electron microscope (magnification 40,000 times), and the crystal grain size at a position 1 μm deep from the inner surface of the steel pipe was measured. Table 2 shows the average values. In addition, the crystal | crystallization in the position of 1 micrometer depth from a steel pipe inner surface means the crystal | crystallization which draws a line segment of length 2 micrometers in the position of 1 micrometer depth from the steel pipe inner surface in each visual field, and crosses or touches the line segment. .

[Steam oxidation test]
(1) Initial heating test A strip-shaped test piece having a thickness of 2 mm, a width of 10 mm, and a length of 25 mm was cut from each specimen so that the inner surface of the tube became a part of the surface of the test piece. The test piece was held in a form suspended from a jig, inserted into a horizontal tubular heating furnace, and subjected to an oxidation test at 650 ° C. for 1000 hours in a water vapor atmosphere having a dissolved oxygen content of 100 ppb. The test piece taken out after the furnace cooling was embedded in a resin, the cross section was cut and mirror-polished, and the oxide scale cross section generated on the inner surface of the steel pipe was observed with an optical microscope to measure the scale thickness. The scale thickness was 500 times, measured in 10 arbitrary fields of view, and the average value of the thicknesses was calculated.

(2) Restoration test The test piece after the initial heating test was repeatedly immersed in an alkali and citric acid bath to chemically remove the oxide scale formed on the surface, and subjected to a simulated peeling treatment. The test piece subjected to the simulated exfoliation treatment was again inserted into a heating furnace, and a steam oxidation test was performed at 650 ° C. × 1000 hours and a dissolved oxygen amount of 100 ppb. After the furnace was cooled, the test piece was taken out, and the generated oxide scale cross section was observed by the same method as described above, and the average value of the oxide scale thickness was calculated.

  In any test, an oxide scale of 15 μm or less was judged to be good.

  As shown in Table 2, No. 1 having a Cr content lower than that required in the present invention. No. 12, a thick oxide scale was formed in any experiment. Although the chemical composition defined in the present invention was satisfied, no surface treatment was performed. 1 and no. In No. 7, a thick oxide scale was formed both after the initial heating test and after the repair test. Although satisfying the chemical composition defined in the present invention, α was outside the range defined in the present invention. 2 and no. In No. 8, the oxide scale after the repair test was thick. In contrast, no. In 3-6 and 9-11, the oxide scale was as thin as 15 μm or less both after the initial heating test and after the repair test, indicating excellent steam oxidation resistance.

  In addition, No. In No. 9, α satisfies the range defined by the present invention, but the crystal grain size at a depth of 1 μm from the surface of the processed layer exceeded 1 μm. In this example, abnormal oxidation was observed on a part of the inner surface of the steel pipe after the initial heating test, and as a result, the scale thickness after the repair test increased.

  Comparing the above results by material, austenitic stainless steel produced the thinnest oxide scale, and the scale thickness increased in the order of ferritic stainless steel and high Cr alloy steel. It is understood that is excellent in steam oxidation resistance.

  According to the present invention, it is possible to uniformly form a scale having excellent protective properties on the surface of the steel pipe in the initial stage of use, and it is possible to suppress the peeling of the scale and even if the peeling of the scale occurs. Since it has a repair function, steam oxidation is unlikely to occur even under high temperature use conditions exceeding 600 ° C. Thus, since the steel pipe of the present invention is excellent in steam oxidation resistance, it is suitable for use in boilers and other high-temperature usage environments.

Claims (5)

A steel pipe containing 8 to 28% by mass of Cr and having a processed layer on its inner surface and satisfying the following formula (1):
α ≧ 0.5 (1)
However, (alpha) in (1) Formula is shown by a following formula.
α = {Hv ₂₀ −Hv _{t / 2} } / Hv _{t / 2}
Here, Hv ₂₀ is Vickers hardness at a position where the depth from the inner surface of the steel pipe is 20 μm, and Hv _{t / 2} is a position where the depth from the inner surface of the steel pipe is t / 2 (t: the thickness of the steel pipe). Each means Vickers hardness.   2. The steel pipe excellent in steam oxidation resistance according to claim 1, wherein the processed layer has a crystal structure having a crystal grain size of 1 μm or less at a depth of 1 μm from the surface thereof.   The steel pipe excellent in steam oxidation resistance according to claim 1 or 2, wherein the steel pipe is composed of a ferritic heat resistant steel pipe containing 8 to 28% by mass of Cr.   3. The steam oxidation resistance according to claim 1, wherein the steel pipe is made of austenitic stainless steel containing 15 to 28% Cr and 6 to 30% Ni in mass%. Excellent steel pipe.   The steel pipe excellent in steam oxidation resistance according to any one of claims 1 to 4, wherein the processed layer is a processed layer formed by spraying particles.